The OSI reference model - · PDF file— World Wide Web Consortium ... Nixu Ltd. 9/35The OSI reference model OSI Layers ... • Unix socket interface example:

Nixu Ltd.
The OSI reference model

Nixu Ltd. 2/35 The OSI reference model
Contents
OSI Reference model
Protocols and services, the difference
Protocol stacks and implementations
What services protocols provide

Layered Protocols
Protocols connect entities on same level, within the layer
Higher layers use services provided by lower layers, adding value
Layers are independent and defined by interface provided to the higher layer and required from the lower layer
Entities communicate by exchanging Protocol Data Units (PDUs) with entities on the same level
Entity N PeerCommunicates with peers
Provides an interface for higher level protocols
Uses services from lower level protocols
following a protocol

Why layers?
Layers may be exchanged as long as interfaces stay unchanged WWW was built on top of existing TCP/IP implementations
WWW does not have to care about the media (modem, Ethernet, radio link etc.)
ATM can be used to transport IP packets without any changes to applications
IPv6 will replace the entire IP part of the TCP/IP layer
Layering makes development easier and adds flexibility

Protocol standardization
International Organization for Standardization (ISO) http://www.iso.ch/
Internet Society & Internet Engineering Task Force http://www.isoc.org/ http://www.ietf.org/
International Telecommunications Union (ITU) http://www.itu.ch/
The Institute of Electronics and Electrical Engineers (IEEE) http://www.ieee.org/
Industry consortiums WAP forum World Wide Web Consortium
De-facto standards

Protocol stacks
Term protocol stack refers to all layers of a protocol family Eg. having TCP/IP stack implemented in an operating system means
that part of the OS software uses device drivers (Ethernet, PPP+serial driver etc.) to receive IP packets and provides socket services to applications programs
Protocol stack has nothing to do with push/pop (data structures)
TCP/IP by IETF is the most popular protocol stack in data communications
Signalling System 7 is the most popular telecoms stack
OSI protocol stack is hardly used in entirety except as a theoretical model (parts of it are in use)
Implementations of protocol stacks often co-exist (OSI X.500 directory system over TCP/IP, TCP/IP communications over telephone network and SS7)

OSI Stack
OSI Internet
7 Application Sovellus
6 Presentation Esitys Application
5 Session Sessio
4 Transport Siirto Transport (TCP & UDP)
3 Network Verkko Network (IP)
2 Data link Linkki Link
1 Physical Fyysinen Physical

OSI Model
Two (N)-level entities in different systems communicate using (N)-level protocol
The services give by layer (N) to layer (N+1) are realized in (N)-level protocol by encapsulation and decapsulation
Encapsulation means embedding each layers Service Data Units (SDU) into the Protocol Data Units (PDU) of the layer immediately below it, decapsulation is the reverse process
Entities at the same level in separate systems are called peer entities (N+1)-level entities are using (N)-level services through (N)-level
Service Access Points ((N)-SAPs) One (N+1)-entity can be simultaneously connected to one or more
(N)-SAPs One (N)-SAP is connected to one (N)-entity

OSI Layers
Physical Layer Defines the physical characteristics of the communications path
Electrical, electromagnetic, optical, acoustic, printed Eg. Ethernet, RS-232
Data Link Layer Transfers frames (packets) over the physical layer Hides the features of the physical implementation Eg. Ethernet, PPP, HDLC
Network Layer Transfers packets over the entire communications network over vari-
ous physical networks Independent of physical implementations Eg. IP, X.25

OSI Layers...
Transport Layer Provides end to end transport of session (application) data Eg. TCP, UDP
Session Layer Provides dialogue (communications path) control Eg. HTTP Connection: keep-alive; TCP urgent pointer
Presentation Layer Provides device independent presentation of data Eg. HTTP Accept: image/gif; MIME headers
Application Layer How applications communicate Eg. HTTP, SMTP...

Internet Model
SNMP, NFS DNS FTP, HTTP, SMTP, Telnet, SSH etc..
UDP TCP
Internet Protocol (IP)
Data Link protocols (Ethernet, PPP, ATM, Frame Relay...)
ICMP
Ping
Physical media

A Data Packet on EthernetApplication
data
ApplicationdataTCP
ApplicationdataTCPIP
ApplicationdataTCPIPETH CRC
ApplicationdataTCPIPETH CRC
ApplicationdataTCPIPPPP
Host Router

Application Level Protocol Sample
220 tcm.hut.fi SMTP/smap Ready.helo jalopeno.nixu.fi250 (jalopeno.nixu.fi) pleased to meet you.mail from: [email protected] [email protected]... Sender Okrcpt to: [email protected] [email protected] OKdata354 Enter mail, end with "." on a line by itselfFrom: [email protected]: [email protected]: terveisi
Heipparallaa.250 Mail acceptedquit221 Closing connection

Service Interface
Protocols are usually internationally standardized
Service interface implementations are usually operating system specific Different implementations can communicate, with common protocol Application programs need to be modified to use different interfaces
Unix socket interface example: 1. open socket 2. parse address 3. connect to server 4. write request 5. read response 6. close socket

What Protocols do?
A protocol shall be: Completely and unambiguously defined Free of dead-locks and live-locks Able to recover from all error conditions
Some possible functions of protocols Addressing Connections and confirmations Error detection and correction Flow control Priorization Multiplexing, segmentation and blocking

Protocol Addressing
On the Internet: People usually use domain names (max 255 chars, eg. www.nixu.fi) IP-protocol uses numeric addresses (four octets, eg. 194.197.118.20) TCP and UDP use 16 bit port addresses (eg. 80), separate address
spaces LANs and data link level protocols have their own addressing (eg. Eth-
ernet, six octets) Application level programs must be able to handle both IP addresses
and port addresses (Unix socket service), but not data link addresses Application level protocols sometimes have their own addresses (e-
mail, URL) Eg. Telnet protocol does not do any addressing
Other Telephone numbers Postal addresses

Connections
Some protocols, like TCP provide a connection from end to end, some, like UDP are connectionless
TCP must store the state and sub-states of the connection No connection Link set-up Data transmission Link disconnect
Three packets to open a TCP connection and four to close
Stateless and connectionless UDP is easier to implement and lighter on the network Applications must worry about data loss, eg. domain name re-
requests

Confirmation
Protocol can provide a receipt
Example: TCP header contains the sequence numbers of traffic UDP itself provides only a single datagram transmission SMTP server acknowledges when an e-mail message is received SNMP traps are not acknowledged and may be lost

Error control
Lowest level transmission channels are analog and somewhat unreliable
Data may be changed, entire frames (packets) may be lost
For reliable communications we need: Error detection Error correction Data retransmission
Having no error control is also option Eg. old modems and terminals

Checksums
A trivial checksum example 1+2+3+4=10, we transmit 1,2,3,4,10 We receive: 1,3,3,4,10 and check 1+3+3+4=11 -> Error detected Our algorithm is not very good, how about: 1,3,2,4,10?
Actual algorithms are more complex CRC-CCITT catches certainly all burst errors of 16 bits or less
And is most likely to catch all other errors, too
Parity bits Odd parity, make data element always to have odd number of ones,
eg: 11100010 -> 111000101
Cryptographic checksums (often called also hashes) make it very hard or impossible to replace or change the data
Checksum design depends on the type of error expected

Error Detection
Checksums can be done at different protocol levels and for different sized chucks of data Parity bit for old terminals IP has header checksum TCP and UDP checksums check both header and payload TCP/IP application level protocols do not usually do error detection,
they trust TCP and UDP Enc

Documents

The OSI reference model - · PDF file— World Wide Web Consortium ... Nixu Ltd. 9/35The OSI reference model OSI Layers ... • Unix socket interface example: